\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}\Omega(f(n)) = {g(n) \exists e, n_0 such that \forall n \ge n_0 0 \le c * f(n) \le g(n)} 
\end{equation*}
\end{document}
